The Myxobacteria are a group of Gram-negative bacteria that undergo a true multicellular development and a primitive differentiation. They provide a simple system from investigating the regulation of development because they resemble more familiar bacteria in structure and in their accessibility to genetic manipulation through mutation and gene cloning. Myxobacteria grow vegetatively like other bacteria, but when nutrients become limiting, Myxobacteria aggregate to form multicellular fruiting bodies. Within nascent fruiting bodies cells sporulate, changing - in the case of Myxococcus xanthus - from long rods to spherical spores, which are dormant. Under appropriate conditions, the spores germinate to give rod-shaped growing cells. Interactions between mycobacterial cells coordinate their development of fruiting bodies. The objective of the research proposed is to find out, in molecular and cellular terms, how cell-cell interactions coordinate this relatively simple developmental process. The signal molecules that cells use when they interact with each other in the formation of fruiting bodies will be isolated and identified. For this purpose, a set of mutants that are defective in producing individual signals will be used as bioassay cells. Receptors for athe signal molecules, and the signal transduction system will be studied using these mutants and a transposable probe for promoter activity. In the long term, this research may facilitate the understanding of human developmental birth defects and speed their medical treatment. Cell-cell interactions are known to be important in human embryonic development as well as in the normal function of the adult human immune system , but developmental interactions in embryos and the immune system of adults are difficult to investigate both technically and ethically. In Myxobacteria the full power of biochemistry and molecular genetics can be used to study its cell interactions. The experimental results may provide a point of departure for investigations in more complex organisms.